But since the donated 7850 is a bit tired, It's possible that it's being close to failing (Can't tell since my current mobo is already broken)

What "tires" out a video card? Did you run it overclocked in a case with bad cooling? If not, then I wouldn't be putting any "artificial" lifespans on that card. Just look at how many people on this forum are still doing 24/7 Folding@Home on nVidia GTX 2xx cards.

Clean the dust off the fan of that 7850, and run MSI Kombustor on that card for half a day - that should be enough to tell you if you should use it in your new build.

__________________"The computer programmer says they should drive the car around the block and see if the tire fixes itself." [src]